Resonant and semi-resonant DC-DC converters, including isolated and non-isolated topologies, are used in a variety of applications including telecommunications, consumer electronics, computer power supplies, etc. The usage of such converters is gaining popularity because of their zero-voltage switching (ZVS) and/or zero-current switching (ZCS) characteristics, and their ability to utilize parasitic electrical properties inherent in an electronic circuit. Among numerous topologies, the semi-resonant converter with transformer/center-tapped inductor is an attractive topology for providing high voltage-conversion ratios without requiring isolation. Such converters provide advantages including lower cost and higher efficiency as compared to other solutions.
One class of semi-resonant converters includes high-side and low-side switches that transfer power from an input source to a center-tapped inductor that supplies output power to a load. The center-tapped inductor is also connected to a second low-side switch, which is termed a synchronous rectification (SR) switch herein. In order to meet the power requirements for a load of a semi-resonant converter (e.g., provide a near constant output voltage for the load), many semi-resonant DC-DC convertersemploy a variable switching frequency wherein the switching period can vary from cycle to cycle. During a portion of each switching period, the SR switch will be enabled such that current flows through it. For the semi-resonant converter described above, the current during this portion of a switching period will be shaped like one half cycle of a sinusoidal period. The time interval for this half-cycle sinusoid is determined by reactive elements within passive circuitry of the semi-resonant converter, e.g., the natural frequency of an inductor/capacitor (LC) resonant tank and other passive components within the semi-resonant DC-DC converter determine this time interval.
It is highly desirable to turn the power switches of a resonant or semi-resonant DC-DC converter on and off when the voltage or current across the relevant switch is at or near zero. Such soft switching has an advantage that switch losses are minimized. Additionally, soft switching avoids electromagnetic interference (EMI) that is due to high-frequency harmonics associated with hard switching. An important consequence of these advantages is that soft-switching resonant and semi-resonant converters can run at much higher efficiencies than their corresponding hard-switching counterparts.
The time interval of the half-cycle sinusoidally-shaped current flowing through an SR switch within a semi-resonant converter determines when the SR switch should be disabled. In order to achieve the desired zero current switching (ZCS), the SR switch should be disabled when this current has returned to zero. The reactive components of the semi-resonant converter determine this time interval. While this time interval may be calculated based upon the inductive and capacitive elements in the circuit, such a calculated time interval will not be perfect due to variations in the reactive elements. More particularly, inductor and capacitor components vary from one to another (as indicated by the tolerance typically assigned to such components), the inherent (parasitic) reactance of the circuit introduces variation, and temperature changes can alter the reactance of some components.
In order to minimize voltage and current ripple at the output of a voltage converter and to scale up its power output, a voltage converter may make use of multiple phases. The phases are each, effectively, separate voltage converters wherein each is tied to a common input voltage source and powers a common output load. To maintain stability and minimize the ripple, the phases should be driven by a common switching frequency, but with the switch control signals to each of the phases staggered in time.
A problem with multi-phase semi-resonant converters is that the time interval of the half-cycle sinusoidally-shaped current will vary from one phase to another due to variations in the inductance and capacitance within each of the phases. A controller using a common (but variable) switching frequency for all of the phases, and staggered versions of a control signal to drive the SR switches for each phase of a semi-resonant converter, will not achieve the ideal zero-current switching (ZCS) described earlier. More particularly, the time interval of the half-cycle sinusoidally-shaped current for some phases of the semi-resonant voltage converter may be relatively short whereas others may be relatively long. This means the controller may disable SR switches for some phases while positive current is still flowing through the SR switches, and may disable other SR switches when negative current is flowing through these SR switches.
In addition to the power loss and EMI associated with switching at a non-zero current, switching an SR switch when negative current is flowing through it may potentially damage the SR switch. Furthermore, negative current flow through an SR switch, as associated with not disabling an SR switch until after negative current is flowing through it, leads to additional power loss as compared with disabling an SR switch while positive current is flowing through it. This is because negative current flow through an SR switch partially discharges the output capacitor(s) of the voltage converter, effectively bleeding energy stored on the capacitor(s) to ground. Disabling the SR switches while positive current is flowing through them does not lead to such discharging of the output capacitor(s) and the associated wasted energy.
Accordingly, there is a need for improved techniques that avoid switching SR switches off in a multi-stage semi-resonant converter when the current flowing through the SR switches is negative.